Hematocrit, the fractional percent volume of erythrocytes (red blood cells) in total volume of blood, is a vital blood constituent to be monitored to determine the physiological condition of a mammalian patient.
Hematocrit has been traditionally measured by withdrawing or diverting blood from the body of a mammalian patient. Such in vitro or in vivo techniques that are invasive to body cause complications in the condition of the patient and those treating the patient. Any invasive diagnostic or monitoring procedure is less preferred by a mammalian patient than a noninvasive procedure. Blood-borne diseases are a source of concern for those treating the patient.
Invasive procedures for determining hematocrit where a sample of blood is withdrawn are disclosed in U.S. Pat. Nos. 3,923,397 (Shuck) and 5,200,345 (Young) and in European Patent Publication 0 419 222 A2 (Callis et al.)
Invasive procedures for determining hematocrit where a sample of blood is diverted from the bloodstream of a patient are disclosed in U.S. Pat. Nos. 4,227,814 (Soodak et al.); 4,303,336 (Cullis); and 4,447,150 (Heinemann); and in European Patent Publication 0 419 223 A2 (Callis et al.).
Invasive procedures for determining hematocrit where a probe is inserted into the body is disclosed in U.S. Pat. Nos. 4,776,340 (Moran et al.) and 4,936,679 (Mersch).
Noninvasive procedures for predicting hematocrit can employ magnetic resonance, as disclosed in U.S. Pat. No. 5,233,991 (Wright); or impedance plethysmography, as disclosed in U.S. Pat. Nos. 4,562,843 (Djordjevich et al.) and U.S. Pat. No. 4,548,211 (Marks).
Noninvasive procedures for predicting percent oxygen saturation employing photoplethysmography, commonly called pulse oximetry, are disclosed in U.S. Pat. Nos. 4,819,752 (Zelin) and 5,193,543 (Yelderman). In Zelin, light emitting diodes at two different wavelengths are employed, with corrections made for the pulsatile nature of blood flow, variations in the strength of the light source, variations in the thickness of tissue through which the light passes, variations in placement of the detector in respect of the location of the light emitters, and changes in the physiology of the non-pulsatile component. In Yelderman, light emitting diodes at two different wavelengths are also employed, with corrections made to improve resistance to interference from ambient artificial light, BOVIE electrocautery interference using AM modulation/demodulation techniques.
Noninvasive procedures for predicting hemoglobin, arterial oxygen content and hematocrit employing photoplethysmography is disclosed in U.S. Pat. No. 5,101,825 (Gravenstein et al.). In Gravenstein et al., the blood constituents are predicted by measuring the change in the mass of hemoglobin resulting from a measured change in the volume of blood. After total hemoglobin is predicted, hematocrit is approximated by multiplying total hemoglobin by a factor of three.
None of the known invasive procedures are preferable to either patient or practitioner if a truly noninvasive technique were available that could predict hematocrit accurately. None of the known noninvasive procedures predict hematocrit directly.
European Patent Publications 0 419 222 A2; 0 419 223 A2 (both Callis et al.); and 0 476 192 A2 (Callis) disclose methods for prediction of hematocrit using chemometric techniques and regression analysis that uses the spectral absorbance of water in the near infrared spectrum to provide the basis of prediction based on a two compartment model where one compartment has a different amount of water than the other compartment. But none of the Publications disclose a method or apparatus to noninvasively predict hematocrit.